This invention relates to an ignition apparatus which is incorporated in a burner using such fuel as oil or gas and serves to provide a stable and secure ignition of the burner.
According to empirical facts, high power electrical spark discharges are needed to provide a secure ignition of fuel in a burner, especially of fuel oil in an oil burner.
The conventional ignition apparatus of this type for a burner consists mainly of a high-voltage step-up transformer and therefore has an advantage that the spark discharge is so intense that ignition may be secured. However, the conventional apparatus also has a drawback that the size is increased which results in an increase in cost.
There is another conventional ignition apparatus which uses thyristors as switching elements. That example is shown in FIG. 1. In FIG. 1, reference numeral 1 indicates an ac power source; 2 a switch; 3 a rectifying diode; 4 a pulse transformer; 5 a commutating diode; 6 a charge/discharge capacitor; 7 a high-voltage step-up transformer; 8 a thyristor; 9 a trigger circuit for the thyristor 8, a consisting of a resistor, a zener diode and a capacitor. This ignition circuit operates as follows. When a voltage higher than a predetermined voltage is developed across the charge/discharge capacitor 6 as a result of charging, the trigger circuit 9 is actuated to turn on the thyristor 8. Accordingly, the charges stored in the charge/discharge capacitor 6 are released through the primary winding of the high-voltage step-up transformer 7 so that a high voltage is induced across the secondary winding thereof. The high voltage then causes spark discharges at the spark gap in the secondary circuit, leading to an ignition of fuel.
With this ignition apparatus, the commutation by the thyristor 8 becomes difficult for a HF current with a frequency higher than 10 KHz and therefore there is a need, in such a case, for providing this circuit with a commutating circuit and a protecting circuit. The resultant apparatus is unstable under the fluctuations of load and input. Moreover, since this circuit comprises an LC circuit consisting of the charge/discharge capacitor and the primary winding of the step-up transformer 7 and is used under a resonant condition, the thyristor 8 must be able to withstand a high voltage. This is a drawback since a thyristor having a higher withstand voltage is more expensive.